Apparatus for finding the course along any definite great circle



' y '1936. J. M. BOYKOW 2,039,878

APPARATUS FOR FINDING COURSE ALONG ANY DEFINITE GREAT CIRCLE Filed Oct. 6, 1954 I NVE'NTOR Patented May 5, 1936 PATENT OFFICE 2,039,878 APPARATUS ron FINDING THE COURSE ALONG ANY DEFINIT'E GREAT CIRCLE Johann Maria Boykow, Berlin-Lichterfelde,

Germany Application October 6, 1934, Serial No. 747,156 In Germany October 7, 1933 12 Claims.

, As is well-known, the shortest way between two points of the earth surface is represented by the great circle .laid around the globe through these two points. A great circle will be obtained by intersecting the surface of the globe by any plane crossing the centre of the globe. The position of any plane being defined by three points and the plane of every great circle crossing the centre of the globe, any two different points on the earth's surface will define one definite great circle, the plane of such circle including a definite angle with the rotation axis of the earth.

If a craft is maintaining a definite course direc tion which does not coincide with a meridian or the equator, such craft will not move along a great circle butalong a loxodromic line, cutting, as is well-known, the meridians at equal angles .and, therefore, having the form of a spiral line terminating at the north pole or the south pole respectively. Hitherto, in order to obtain a course nearly along a definite great circle connecting the point of departure with the point of destination, the course is from time to time varied in such away that the whole travel-line consists of loxodromic parts and thus approximates the desired great circle.

The object of this invention is to provide an apparatus by which the great circle connecting the point of departure with the point of destination may be actually found, so that a craft equipped with such an apparatus can continuously move along the desired great circle. I For this reason the new apparatus may also be called a great circle finder.

The main principle of the invention consists in utilizing the fact that the position of the polar axis of the travel path in relation to the terrestrial globe will be maintained constant as long as the craft is travelling along, the predetermined great circle.

Other objects will appear hereinafter. It will be evident that many embodiments of the invention may be found. In the preferred form there are provided two elements or members, of which the one serves for reproducing the position of the rotation axis of the earth and the other for reproducing the position of the polar axis of the great circle travel path. As regards-these elements and their cooperation there are many possibilities. A very exact and simple embodiment is obtained by controlling and supervising the positions of the two said'elements by relating them to an astronomical fixed point, for instance the sun. As regards this embodiment the two elements or members reproducing the positions of the polar axis of the great circle and the position of the rotation axis of the globe are constituted by the rotation axes ofa sight which, in using the apparatus, is to be maintained directed to said astronomical fixed point. To this end the sight is driven by a constant speed motor, for instance a spring motor, around that axis which is parallel to the axis of the globe with an angular velocity which is equal to the angular velocity of. the earth.

In consequence of this, the sight having been properly adjusted will be maintained directed to the astronomical fixed point, unless the apparatus is moved in relation to earth. When new the apparatus is travelling along a great circle the conditions, as regards the adjustment of the apparatus in relation to the astronomical fixed point, are equal to those that have been maintained when the apparatus is at rest in relation to the earth but the earth would additionally rotate about the polar axis of the great circle (apparent earth rotation). If now this apparent rotation of the earth is also compensated the sight will necessarily be maintained directed to the astronomical fixed point. The said apparent earth rotation is, as regards its value, equal to the speed of the craft equipped with the said apparatus. If this value is measured in any suitable way and if according to this value the sight is rotated about the axis parallel to the polar axis of the great circle travel path, the sight will be maintained directed to the astronomical fixed point as long as the craft is moving along the great circle. In case the image of the astronomical fixed point leaves the zero point of the cross wires of the focus plane belonging to the sight, this indicates that the craft has deviated from the great circle. Therefore, in order to move along the great circle the craft is to be steered in such a manner that the astronomical fixed point or, more exactly, the image of the astronomical fixed point will be maintained at the zero point of one of the cross wires.

For the purpose of a better explanation it has been assumed above that besides the turning of the sight according to the actual earth rotation, the sight is also continuouslyturned in such a manner as to compensate the apparent earth rotation. A continuous compensating of the apparent earth rotation and an exact measurement of the speed of the craft or any measurement of the speed at all is not required as will be seen from thefollowing descriptive drawing.

the surface of the 15 For a more complete understanding of the invention reference should be had to the accompanying drawing.

Fig. 1 is an elevational and in part a sectional view of an apparatus embodying the invention.

Fig. 2 shows some elements of the embodiment illustrated in Fig. 1, but on an enlargedscale to make the detail features of these elements clear.

Fig. 3 is a vertical sectionalview taken-approximately in the plane of line III-I1I of Fig. 2.

- Fig. 4 is an elevational detail view of the focus plane element of the embodiment illustrated in Fig. l, in the field of which focus plane element some positions of the image of the astronomical fixed point being shown for the sake of better 7 explanation.

Fig. 5 is a geometrical diagram showing the 'surface of the terrestrial globe and on this globe a great circle along which an aircraft equipped with an apparatus embodying this invention is travelling.

Fig. 6 is a perspective view of an auxiliary deviceserving for determining the angle between the polar axis of a definite great circle and the rotation axis of the earth.

Fig.7 is a perspective view of a modified embodiment of the auxiliary device shown in Fig. 6.

Referring now to Fig. 1, P is a base or the platform which, in general, is to be stabilized in a horizontal plane in any manner, for instance, by means of a well-known gyroscope system. The apparatus embodying the invention is mounted on this base P. The said. apparatus comprises a vertical tubing l angularly adjustable around a vertical axis by means of a wormgear 2 and a hand crank 3. The adjustment of the .tubing I is indicated by the indicating device 4, coupled with the said handle 3. On the tubing I or, more exactly, on a horizontal lateral part of this tubing a tubing 5 is rotatably mounted, angularly adjustable around a horizontal axis HH by means of a worm gear 6 and a handle 1 according to the readings on a scale 9, coupled with the said-handle' A tubing 9 in turn is mounted pivotally, for instance as shown by means of hollow pivot members 5a,

9a of cylindrical curvature, on the said horizontal tubing 5 and the tubing 9 respectively. The adjustment of this tubing 9 in relation to the tubing 5 is achieved by means of a hand knob "I, the bearing of which is carried by a support 5b rigidly fixed to the tubing 5. On the axis of the said knob III a toothed wheel I l is mounted, mesh: ing with a toothed wheel segment 9b of the pivot part 90.. The adjusted angle, 1. e. the angle between the tubing 9 or, more exactly, the geometric axis E-E of this tubing 9 and the tubing 5, or better, the geometric axis HH of this tubing, is indicated by the pointer I 0a fixed to the handknob ID on the scale l2 which is provided on the support 5b. By means of a connecting tubing l3 rotatably mounted on the 'tubing 9,

the latter carries a sight I, rotatable in relation to the tubing IS.

The said sight I is rotatable around the geometrical longitudinal axis E-E of the tubing 9 by means of the spur gear system l5 coupled with a springmotor l6 fastened to the tubing 9. The field covered by the sight I! is projected by means of the optical system mounted in the tubings I, 5, 9 and I3 upon the image or focus plane H, where there is a glass plate or the like with cross wire: Ila and Nb (see Fig. 4).

The optical system shown in the drawing angle of comprises a mirror I8 mounted in the sight I, which mirror is to be adjusted around the axis I8a according to the declination of the astronomi- .ratio of these two gears is such that the tubing.

13 will rotate at half the angular velocity of the sight i4, so that, if the sight turns through an angle of 360, the'tubing 13 turns through an Within the tubing 9 the objective 2| is mounted. In the image or focus plane of this objective 2| within the cavity formed by the pivot 5a, 9a there is a mirror 22 fastened to the pivot part 9a. From this mirror the rays are reflected to the mirror 23, which is mounted to turn about the axis of a pin 23a journaled in the pivot part 5a.

It will easily be understood that the mirror 23 must be turned at half of that angular velocity at which the tubing 9 and, at the same time, the v mirror 22 are turned in relation to the tubing 5, thus. enabling the reflected beam from the mirror 25 to maintain its direction parallel to the tubing 5. It is preferred to couple the mirror 23 with the hand knob III for an automatic adjustment of said mirror when the tubing 9 is turned about the pivot 5a, 9a. To this end a spur gear system 24 25 is provided inthe embodiment illustrated by the drawing, of which spur gear the toothed wheel 24 is secured to the pivot pin 23a of the mirror 23 and the toothed wheel. 25 is fastened to the spindle of the hand knob Ill. The transmission ratio of this spur gear is according to the above explanation suchthat the mirror 23v will turn at half of that angular velocity, at which the tubing 9 and the mirror .22 fastened to this tubing 9 are rotated when the hand knob I0 is turned. 'From the Y mirror 23 the path of the rays is through the 53 meshing with a toothed wheel 54 rotatably I journaled on the tubing 5 at one side thereof. This toothed wheel 54 in turn meshes with internal teeth on acylindrical ring 55 constituting, as regards its exterior, the wheel of'a worm gear 56, the worm of which is coupled by means of its shaft and a spur gear system 51 with the shaft .of the hand crank I. The transmission ratio 01' the said gears is such that the Doveprism 21, as said above, will follow the angular movement. of the tubing 5 with half the angular velocity of this tubing 5.

In order to obtain a clearer illustration, in Fig. 1 the tubing l1 containing the focus plane is shown as being parallel to the tubing 5, but I desire it to be understood that I do not wish to restrict myself to this particular arrangement.

It will be clear to those skilled in the art that the optical system shown in the drawing may be replaced by any other suitable optical system. This remark applies particularly-to the so-called optical pivot 23a, 22 within the cavity formed by the mechanical pivota, 9a. The tubing 5 or, more exactly, its geometrical axis H-H represents that rotation axis of the sight M which is to be positioned according to the axis of the great circle travel path of the craft carrying this apparatus. The tubing 9 or, more exactly, its geometrical axis EE represents that rotation axis of the sight M which is to be positioned according to the rotation axis ofthe terrestrial globe. Accordingly, the spring motor I6 serves for, so to speak, turning back the sight i4 according to the actual earth rotation. In consequence of the fact that the tubing 5 or, more exactly, its geometrical axis H--H represents that axis which is to be positioned according to the axis of the great circle travel path of the craft, the apparatus is to be adjusted on this craft in such a way that the axis H-H within the horizontal plane will be at right angles to the direction of the travel path of the craft. Therefore, in general, 1. e. in case no side slip arises, the axis HH will be at right angles to the longitudinal axis of the craft. At the beginning of the travel, the tubing 9 is to be adjusted in relation to the tubing 5 in such a manner that the angle between the two tubings 5 and 9 or, more exactly, the angle between the geometrical axes HH and EE of these tubings is equal to the angle between the rotation axis of the terrestrial globe and the polar axis of the great circle along which the craft is to be moved.' This adjustment is then not to be changed any more during the travel of the'craft along this particular great circle.

For a better explanation of the mode of operation, reference may be had to Fig. 5. In this figure the terrestrial sphere with its rotation axis N-S and its equator A is shown. G represents a great circle, along which the aircraft F diagrammatically shown may be supposed travelling. G designates the polar axis of said great circle G. By the term polar axis I means a line passing through the centre of the great circle and perpendicular to the plane of said c rcle. In Fig. 5, as regards the embodiment shown in Fig. 1, only the sight l4 and the two geometrical axes EE and H-H are diagrammatically shown. As mentioned above, at the beginning of the travel of the craft, the tubings 5 and 8 are, in respect to each other, adjusted in such a manner that.

i If now, furthermore, the sight M has been angularly adjusted about the axis EE according .to

the local or meridian time for which purpose a scale l3a. may be provided on the axis E-E or, more exactly, on the tubing l3, and a co-operating pointer Ma. on the sight I4, the image of the astronomical fixedpoint, for instance of the sun. will be visible in the centre of the cross-wires Na and Nb of the focus plane H (see Fig. 4). If the craft F is travelling along the great c'rcle G, the conditions regarding the variation in the position of the great circle finder due to the movement of the craft along the great circle are equal to those that would obtain if the'craft were in the centre of the globe and would rotate about its transverse axis, parallel to the axis H-H. If, therefore, for better understanding of the operation, the craft F is imagined as shifted parallel to itself into the centre 0 of the globe, it will be easily seen that one complete rotation of the craft F about its transverse axis corresponds to a travel of the craft F along the great circle 6 around the globe and that during this rotaalong the whole great circle G around the globe,

the sight I will perform a complete rotation in relation to the craft. It will be easily understood that in this way the axis EE will be maintained parallel to the rotation axis NS of the earth. If the follow-up movement of the sight l4 about the axis H-H should be interrupted for a certain length of time, the image of the astronomical fixed point in the focus plane l'l. will leave the wire l'lb. From this it results that, in order to obtain the proper follow up" movement of the said member, the operator need only take care to havethe image of the astronomical fixed point remain on the wire l'lb.

Any deviation of the craft from the line of the great circle G which is 'to be travelled by the craft results regarding the position of the apparatus in conditions equal to those as would be obtained in displacing again the craft to the centre 0 of the globe and turning it about its vertical axis out of the plane through the great circle G. 7

It will be easily realized that such a turning of the craftwhich may correspond to the assumed deviation from the great circle to be travelled, will necessarily result in a displacementof the image of the astronomical fixed point along the direction of or parallel to the wire llb. Therefore it can be determined from the position of the image within the field of the cross wires Ila and Nb whether the craft moves on the great circle to be travelled and also whether the sight I has been' properly followed up by rotation about the axis H--H. So if, for instance, at a certain point of time the image of the astronomical fixed point within the field of the cross-wires "a and I 1b is at the point B shown in Fig. 4, it will be evident from this position B that the sight II has not sufiiciently turned about the axis I-I-H, So the operator will have to turn the sight l4 about the axis H-H until the image of the astronomical fixed point arrives at the position 31. Thereupon the craft will have to be steered so that the image of the astronomical fixed point moves along the wire Nb and arrives-at the intersecting point of the two wires I'Ia and lib, i. e. the position B2. In the same way theoperator may proceed in or presently upon starting the craft. In case of an aircraft, for instance, on ascending of the craft, the operator will first turn the craft about its upright axis until the image of the astronomical point becomes visible within the field of the cross wires .45 ;1mzand znbwu'hezsubsequens can also :be compensated: aivery simple man--= I nor. this: purpose an auxiliary I device :may I f 'bc fused, which Eprinits to determine :the: angle dent that theiangle through which; the sight M I turned aboutsthe axis 'H-i -Hi-aiter the; initial I f adjustment: at the beginning of: traveI -i-for corn 1 i pensating: the apparent: earth: rotation,- :i.= e=..che

movement; or the craitrelative to the earizh, af-z :ford's: an: immediate measure :ior: the 1 length: of

; the itravel :path covered; I :The' :new apparatus I r i may therefore at the; same time be used as a meter; for :the length of i thei travel path. Furthermore from: the angular velocity i with which :the; is 'to P be turned for Ii compensating f the apparent earth rotation ithe I I over ground speed of the: craft nlay: be: derived;

' Consequent1y thei new: apparatus he also I applied I for: determining the; said: speed: over 1 I :In the above the; influenceoi :side drift caused; i for instancaby iwater or-air currents :has not yetibesn takenE in consideration. iiEnfl-liel 'io between the polar axis of the gi'eat circle travel seen from the German; Patent No; 513,546.: :A'c I cording 150 :said: angle: the; great: icircie finde'r will I I Bath and the: craitzsi transverse: Devices of: this kind aic-i viiell: hiowny as: warm: for insfianc hemmed; by :means of the hand crank 3 around ts: vertical axis. i Irwin be easily understoodathat the: craft :will move along the: great-circle i in:

' :spite' f thfside drift; the il'mw apparatus properly operated and used in: the manner diescribed above. I The angle needed ion :the: ad-

'ustment OI: thfi apparatus: according to 'the angle I between the; rotationiaxis ofthe earth and rho I :polar; axis: of the I desired; great circle: may ibe; I :cietermincd by: a calculation from the gees-rabbi:

I I cal data of the pointof: departure and the data of the point of destination. In a more simple manner the said angle may be obtained by means of the auxiliary device illustrated in Fig. 6. This device comprises a globe 40 having meridian lines and latitude circles. A bow 42 is rotatably mounted about an axis 4| fastened to the globe, and on this how 42 in turn a hemisphere 43 is rotatably mounted. The ads about which the hemisphere 43 is rotatable in relation to the bow 42 and the axis about which the bow is rotatable in relation to the globe 40 intersect at right an gles in the centre of the globe 40.

In the position shown in Fig. 6 the upper edge of the hemisphere 43 coincides with the equator line of the globe 40.. The hemisphere 43 bears a scale 43a, on which its inclination is indicated "by the hand 42a. The mode of operation of this auxiliary device is as follows:

If, for instance, the great circle connecting the point 01 with the point 02 or, more exactly, the angle betweendts polar axis and the rotation axis of the earth is to be determined, the hemisphere 43 by turning in relation to the bow and,

if necessary, by turning the bow 42 in relation to the globe, is brought into such a. position that its edge will pass through the two points 01 and akei ; greataltitudes :there is practically no: visibilit down to the I'earth and :for I; this reason: no .Possi bility: of: finding onczs :way by: observing points on :the: earth; The: same remark: holds :tor: tTHHS' i I marine flights and, therefore, the new apparatus affords a remarkable progress for such flights.

the sky is not visible.

image or: focus field: l is: mounted onithe tubing;

. t or, more exactly, on a tubing branchediromi nsteiad of this; the iocus field might; be provide or aVigating enemas performing flights a ei:

the: like; will in most cases: be unnecessary. i In steadcfi stabilizing thesntire apparatus, it: wi be sufiicient to stabilize meirely the? focus: fiel .i :or; more exactly, the ero'sswilles' :la'iaand I? in general, in performing ililglitsatcomparativel Finally the new apparatus may, of course, be used on aquatic or land crafts. As far as the latter are concerned, ridesthrough bleak, uninhabited areas, for instance, deserts, are examples of conditions where the use of my invention will prove of special value.

As an astronomical fixed point, according to which the new apparatus is to be adjusted, in the above the sun was mentioned.v Instead of the sun any other astronomical fixed point can be used. The fact phat astronomical fixed points cannot be seen from the earth when the sky is overcast, is without significance in performing high altitude flights, since they take place above the clouds in a region-where the celestial bodies are visible at all times. I

"The new apparatus may be used even when In this case, however, an auxiliary device is necessary which determines the position of that astronomical fixed point according to which the apparatus is adjusted. If,.

for example, the sun is used as the astronomical fixed point it will be possible to determine the position of the sun by means of .a bolometer or the like responding to the invisible radiation of the sunpenetrating the clouds. If this bolometer or other measuring instrument is positioned according to the position of the sun, 1. e. if the said instrument shows the direction of the sun in relation to the craft, and if in turn according to this instrument the great circle finder is adjusted, it

traordinary: heights, for instance, :up toithe: strat'r oSphr. l iin such ihigh flights :the; aircraft;

; I :IYhe-great practical fvallleffiffth new. apparatus. ecom'cs' obvious upon considering the iact 5 that.

am of a circle; i As this indicator iolows thecu'r' 'valzure :of Ii the globe 4051:1318 globe encased :by he member; 45.5 :In' using ide'vice' the ihsm sphier'e roflafvediin relation to the globe lil unt I its edge i connects the point of ildeparturei with; I I the point; of destination; :'I'he= position of the i indicai'nr point; 45w within iahescale constitutedby the II iatitude-circies will then show the mag 01 be'idct'erm inedf as between she polar :axis of .1

will be possible to use the great circle finder even when the sky is overcast,

I wish it to be understood that although in order to make clear the nature of my invention I have described certain specific examples thereof I ing a sight movable abouta horizontal axis and also about an axis intersecting the first-mentioned axis, means for rotating said sight about the first-mentioned axis in accordance with the movement of the instrument relatively to the earth, means for rotating the sight about the second-mentioned axis in accordance with the rotation of the earth about its own axis, and means for moving said sight to alter the angle between said two axes and make it equal to the angle between the earths axis and a line perpendicular to the plane of the great circle path to be traveled,

at the center of such great circle.

2. An apparatus for finding'a definite great circle travel path containing an optical sight adapted to be directed to an astronomical fixed point, a support rotatable about a vertical axis, a tubing at right angles to the said support and rotatably mounted on it to turn about a horizontal axis, a second tubing, mechanical means pivotally conmeeting the first-named tubing with the second tubing to permit the latter to turn relatively to the former about an axis transverse to both of said axes, the said optical sight being mounted on the second tubing and rotatable about the geometrical axis of this tubing, and an optical system located within the tubings and co-operating with the said optical sight, the said optical system comprising an optical pivot arranged within the mechanical means pivotally connecting the said second tubing with the first-named tubing.

3. A great circle finding instrument, comprising a base, a carrier rotatable relatively thereto about a vertical axis, a supporting member mounted on said carrier and rotatable relatively thereto about a horizontal axis, a holder supported by said member and mounted to turn relatively thereto about an axis transverse to both of said first-mentioned axes, a sight carried by said holder and rotatable relatively thereto about an axis transverse to the holders axis of rotation and. intersecting the said horizontal axis, means for rotating said sight about its axis of rotation in accordance with the rotation of the earth about its own axis, and separate means for rotating the carrier relatively to its base, the supporting member relatively to said carrier, and the holder relatively to said member.

4. A great circle finding instrument, comprising a carrier, at supporting member mounted on said carrier and rotatable relatively thereto about a horizontal axis, a holder supported by said member and mounted to turn relatively thereto about an axis transverse to the first-mentioned axis, a sight carried by said holder and rotatable relatively thereto about an axis transverse to the holders axis-of rotation and intersecting said horizontal axis, means for rotating said sight about its axis of otation in accordance with the rotation of the earth about its own axis, and separate means for rotating the supporting member relatively to said carrier, and the holder relatively to said member.

5 An instrument according to claim 4, in which the carrier is provided with an optical observation device, and in which an optical system directs light from said sight to said observation device.

6. An instrument according'to claim 4, in which the carrier is provided with an optical observation device, and in which an optical system composed of elements mounted on the carrier, the supporting member, and the holder, directs light from said sight to said observation device;

'7. An instrument according to claim 4, in which the carrier, the supporting member, the holder, and the sight are constructed as tubes within which are located the elements of an optical system for directing light from said sight to an optical observation device on the carrier.

8. An instrument according to claim 4, in which the carrier, the supporting member, the holder, and the sight are constructed as tubes within which are located the elements of an optical system for directing light from said sight to an optical observation device on the carrier, said elements including a prism interposed between other of said elements, and means for rotating such prism at half the speed at which the adjacent element is rotated,

9. An instrument according to claim 4, in which the carrier, the supporting member, the holder, and the sight are constructed as tubes within which are located the elements of an optical system for directing light from said sight to an optical observation device on the carrier, said elements including two mirrors in mutually reflecting relation, one ofsaid mirrors being secured rigidly to said holder, and the other pivoted to said supporting member, and an operative connection for automatically turning said last-mentioned mirror on its pivot at one-half the angular rate at which the holder swings relatively to said supporting member.

10. A great circle finding instrument, comprising a sight movable about a horizontal axis and also about anaxis intersecting the firstmentioned axis, optical means adjustably mounted in said sight and adapted to be inclined relatively to said second-mentioned axis, means for rotating said sight about the first-mentioned axis in accordance with the movement of the instrument relatively to the earth, means for rotating the sight about the second-mentioned axis in accordance with the rotation of the earth about its own axis, and means for moving said sight to alter'the angle between said two axes and make it equal to the angle between the earths axis and a line perpendicular to the plane of the great circle path tobe travelled, at the center of such great circle.

11. A great circle finding instrument, comprising a sight movable about a horizontal axis and also about an axis intersecting the firstmentioned axis. means for rotating said sight about the first-mentioned axis in accordance with the movement of the instrument relatively to the earth, means for rotating the sight about the second-mentioned axis in accordance with the rotation of the earth about its own axis, said last-mentioned means including automatic operating means connected with said sight and operating at a constant speed such that said sight will make one complete rotation about said second-mentioned axis in twenty-four hours in a direction opposite to that of the earths rotation, and means for moving said sight to alter the angle between said two axes and make it equal to the angle between the earths axis and a line perpendicular to the plane of the great circle path to be travelled, at the center of such great circle.

12. A great circle finding instrument, comprising a sight movable about a horizontal axis and also about an axis intersecting the first-mentioned axis, optical means adjustably mounted in said sight and adapted to be inclined relatively to said second-mentioned axis, means for rotating said sight'about the first-mentioned axis in accordance with the movement of the instrument relatively to the earth, means for rotating the sight about the second-mentioned axis in accordance with the rotation of the earth about its own axis, said last-mentioned means including automatic operating means connected with said sight and operating at a. constant speed such that said sight will make one complete rotation about said second-mentioned axis in twenty-four hours in a direction opposite to that of the earth's rotation, and means for moving said sight to alter the angle between said two axes and make it equal to the angle between the earths axis and a line perpendicular to the plane of the great circle path to he travelled, at the center of such great circle.

JOHANN MARIA BOYKOW. 

